Not to worry, we've got 3129 and 5558. Let's get to adding them!

Step 1: We'll start by adding the digits 9 & 8 in column 1 and get 17.
We'll write down the last digit 7 and carry the 1 to the next column.

Step 2: We'll start by adding the digits 2 & 5 in column 2 and get 8.

Step 3: We'll start by adding the digits 1 & 5 in column 3 and get 6.

Step 4: We'll start by adding the digits 3 & 5 in column 4 and get 8.

So, 3129 + 5558 = 8687.

